Systems and Methods for Output Current Regulation in Power Conversion Systems

ABSTRACT

Systems and methods are provided for regulating power conversion systems. A system controller includes: a first controller terminal configured to receive a first signal related to an input signal for a primary winding of a power conversation system; and a second controller terminal configured to output a drive signal to a switch to affect a current flowing through the primary winding, the drive signal being associated with a switching period including an on-time period and an off-time period. The switch is closed (e.g., being turned on) in response to the drive signal during the on-time period. The switch is opened (e.g., being turned off) in response to the drive signal during the off-time period. A duty cycle is equal to a duration of the on-time period divided by a duration of the switching period. The system controller is configured to keep a multiplication product of the duty cycle and the duration of the on-time period approximately constant.

1. CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No.201510249026.4, filed May 15, 2015, commonly assigned, incorporated byreference herein for all purposes.

2. BACKGROUND OF THE INVENTION

Certain embodiments of the present invention are directed to integratedcircuits. More particularly, some embodiments of the invention providesystems and methods for regulating output currents. Merely by way ofexample, some embodiments of the invention have been applied to powerconversion systems. But it would be recognized that the invention has amuch broader range of applicability.

Light emitting diodes (LEDs) are widely used for lighting applications.Oftentimes, approximately constant currents are used to control workingcurrents of LEDs to achieve constant brightness. FIG. 1 is a simplifieddiagram showing a conventional LED lighting system. The LED lightingsystem 100 includes a system controller 102, resistors 104, 124, 126 and132, capacitors 106, 120 and 134, a diode 108, a transformer 110including a primary winding 112, a secondary winding 114 and anauxiliary winding 116, a power switch 128, a current sensing resistor130, and a rectifying diode 118. The system controller 102 includesterminals (e.g., pins) 138, 140, 142, 144, 146 and 148. For example, thepower switch 128 is a bipolar junction transistor. In another example,the power switch 128 is a MOS transistor. In yet another example, thepower switch 128 is an insulated-gate bipolar transistor.

An alternate-current (AC) input voltage 152 is applied to the system100. A bulk voltage 150 (e.g., a rectified voltage no smaller than 0 V)associated with the AC input voltage 152 is received by the resistor104. The capacitor 106 is charged in response to the bulk voltage 150,and a voltage 154 is provided to the system controller 102 at theterminal 138 (e.g., terminal VCC). If the voltage 154 is larger than apredetermined threshold voltage in magnitude, the system controller 102begins to operate normally and generates a drive signal 199 through theterminal 142 (e.g., terminal GATE). The switch 128 receives a signal 156associated with the drive signal 199. For example, the drive signal 199is a pulse-width-modulation (PWM) signal with a switching frequency anda duty cycle. The switch 128 is closed (e.g., being turned on) or open(e.g., being turned off) in response to the drive signal 199 so that theoutput current 158 is regulated to be approximately constant.

The auxiliary winding 116 charges the capacitor 106 through the diode108 when the switch 128 is opened (e.g., being turned off) in responseto the drive signal 199 so that the system controller 102 can operatenormally. For example, a feedback signal 160 is provided to the systemcontroller 102 through the terminal 140 (e.g., terminal FB) in order todetect the end of a demagnetization process of the secondary winding 114(e.g., for charging or discharging the capacitor 134 using an internalerror amplifier in the system controller 102). In another example, thefeedback signal 160 is provided to the system controller 102 through theterminal 140 (e.g., terminal FB) in order to detect the beginning andthe end of the demagnetization process of the secondary winding 114. Theresistor 130 is used for detecting a primary current 162 flowing throughthe primary winding 112, and a current-sensing signal 164 is provided tothe system controller 102 through the terminal 144 (e.g., terminal CS)to be processed during each switching cycle. Peak magnitudes of thecurrent-sensing signal 164 are sampled and provided to the internalerror amplifier. The capacitor 120 is used to maintain an output voltage168 so as to keep a stable output current through an output load (e.g.,one or more LEDs 122). For example, the system 100 implements aprimary-side-regulation scheme with single-stage power factor correction(PFC). As an example, the system 100 implements a flyback architectureor a buck-boost architecture.

FIG. 2 is a simplified conventional diagram showing the systemcontroller 102 as part of the system 100. The system controller 102includes a ramp-signal generator 202, an under-voltage lock-out (UVLO)component 204, a modulation component 206, a logic controller 208, adriving component 210, a demagnetization detector 212, an erroramplifier 216, and a current-sensing component 214.

As shown in FIG. 2, the UVLO component 204 detects the signal 154 andoutputs a signal 218. If the signal 154 is larger than a firstpredetermined threshold in magnitude, the system controller 102 beginsto operate normally. If the signal 154 is smaller than a secondpredetermined threshold in magnitude, the system controller 102 isturned off. The second predetermined threshold is smaller than or equalto the first predetermined threshold in magnitude. The error amplifier216 receives a signal 220 from the current-sensing component 214 and areference signal 222 and outputs an amplified signal 224 to themodulation component 206. The modulation component 206 also receives asignal 228 from the ramp-signal generator 202 and outputs a modulationsignal 226. For example, the signal 228 is a ramping signal andincreases, linearly or non-linearly, to a peak magnitude during eachswitching period. The logic controller 208 processes the modulationsignal 226 and outputs a control signal 230 to the driving component 210which generates the signal 199 to turn on or off the switch 128. Forexample, the demagnetization detector 212 detects the feedback signal160 and outputs a signal 232 for determining the end of thedemagnetization process of the secondary winding 114. In anotherexample, the demagnetization detector 212 detects the feedback signal160 and outputs the signal 232 for determining the beginning and the endof the demagnetization process of the secondary winding 114. Inaddition, the demagnetization detector 212 outputs a trigger signal 298to the logic controller 208 to start a next cycle. The system controller102 is configured to keep an on-time period associated with themodulation signal 226 approximately constant for a given output load.

The system controller 102 is operated in a voltage-mode where, forexample, the signal 224 from the error amplifier 216 and the signal 228from the oscillator 202 are both voltage signals and are compared by thecomparator 206 to generate the modulation signal 226 to drive the powerswitch 128. Therefore, an on-time period associated with the powerswitch 128 is determined by the signal 224 and the signal 228.

FIG. 3 is a simplified conventional diagram showing the current-sensingcomponent 214 and the error amplifier 216 as parts of the systemcontroller 102. The current-sensing component 214 includes a switch 302and a capacitor 304. The error amplifier 216 includes switches 306 and308, an operational transconductance amplifier (OTA) 310.

As shown in FIG. 3, the current-sensing component 214 samples thecurrent-sensing signal 164 and the error amplifier 216 amplifies thedifference between the signal 220 and the reference signal 222.Specifically, the switch 302 is closed (e.g., being turned on) or open(e.g., being turned off) in response to a signal 314 in order to samplepeak magnitudes of the current-sensing signal 164 in different switchingperiods. If the switch 302 is closed (e.g., being turned on) in responseto the signal 314 and the switch 306 is open (e.g., being turned off) inresponse to the signal 232 from the demagnetization detector 212, thecapacitor 304 is charged and the signal 220 increases in magnitude. Ifthe switch 306 is closed (e.g., being turned on) in response to thesignal 232, the switch 308 is open (e.g., being turned off) in responseto a signal 312 and the difference between the signal 220 and thereference signal 222 is amplified by the amplifier 310. The signal 312and the signal 232 are complementary to each other. For example, duringthe demagnetization process of the secondary winding 114, the signal 232is at a logic high level and the signal 312 is at a logic low level. Theswitch 306 remains closed (e.g., being turned on) and the switch 308remains open (e.g., being turned off). The OTA 310, together with thecapacitor 134, performs integration associated with the signal 220. Inanother example, after the completion of the demagnetization process ofthe secondary winding 114, the signal 232 is at the logic low level andthe signal 312 is at the logic high level.

Under stable normal operations, an average output current is determined,according to the following equation (e.g., without taking into accountany error current):

$\begin{matrix}{\overset{\_}{I_{o}} = {\frac{1}{2} \times N \times \frac{V_{ref\_ ea}}{R_{cs}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where N represents a turns ratio between the primary winding 112 and thesecondary winding 114, V_(ref) _(_) _(ea) represents the referencesignal 222 and R_(cs) represents the resistance of the resistor 130. Asshown in Equation 1, the parameters associated with peripheralcomponents, such as N and R_(cs), can be properly selected throughsystem design to achieve output current regulation.

For LED lighting, efficiency, power factor and total harmonic are alsoimportant. For example, efficiency is often needed to be as high aspossible (e.g., >90%), and a power factor is often needed to be greaterthan 0.9. Moreover, total harmonic distortion is often needed to be aslow as possible (e.g., <10%) for some applications. But the system 100often cannot satisfy all these needs.

Hence it is highly desirable to improve the techniques of regulatingoutput currents of power conversion systems.

3. BRIEF SUMMARY OF THE INVENTION

Certain embodiments of the present invention are directed to integratedcircuits. More particularly, some embodiments of the invention providesystems and methods for regulating output currents. Merely by way ofexample, some embodiments of the invention have been applied to powerconversion systems. But it would be recognized that the invention has amuch broader range of applicability.

According to one embodiment, a system controller for regulating a powerconversion system includes: a first controller terminal configured toreceive a first signal related to an input signal for a primary windingof a power conversation system; and a second controller terminalconfigured to output a drive signal to a switch to affect a currentflowing through the primary winding of the power conversion system, thedrive signal being associated with a switching period including anon-time period and an off-time period. The switch is closed in responseto the drive signal during the on-time period. The switch is opened inresponse to the drive signal during the off-time period. A duty cycle isequal to a duration of the on-time period divided by a duration of theswitching period. The system controller is configured to keep amultiplication product of the duty cycle and the duration of the on-timeperiod approximately constant.

According to another embodiment, a system controller for regulating apower conversion system includes: a ramp-current generator configured toreceive a modulation signal and generate a ramp current based at leastin part on the modulation signal; a ramp-signal generator configured toreceive the ramp current and generate a ramping signal based at least inpart on the ramp current; a modulation component configured to receivethe ramping signal and generate the modulation signal based at least inpart on the ramping signal; a driving component configured to receivethe modulation signal and output a drive signal to a switch to affect acurrent flowing through a primary winding of a power conversion system,the drive signal being associated with a switching period including anon-time period and an off-time period. The switch is closed in responseto the drive signal during the on-time period. The switch is opened inresponse to the drive signal during the off-time period. A duty cycle isequal to a duration of the on-time period divided by a duration of theswitching period. The ramp-current generator is further configured togenerate the ramp current approximately proportional to the duty cyclein magnitude.

According to yet another embodiment, a system controller for regulatinga power conversion system includes: a first controller terminalconfigured to provide a compensation signal based on at leastinformation associated with a current flowing through a primary windingof a power conversion system; a ramp-current generator configured toreceive a modulation signal, the compensation signal and a firstreference signal and generate a ramp current based at least in part onthe modulation signal, the compensation signal and the first referencesignal; a ramp-signal generator configured to receive the ramp currentand generate a ramping signal based at least in part on the rampcurrent; a modulation component configured to receive the ramping signaland the compensation signal and generate the modulation signal based atleast in part on the ramping signal and the compensation signal; and adriving component configured to receive the modulation signal and outputa drive signal to a switch to affect the current, the drive signal beingassociated with a switching period including an on-time period and anoff-time period. The switch is closed in response to the drive signalduring the on-time period. The switch is opened in response to the drivesignal during the off-time period. A duty cycle is equal to a durationof the on-time period divided by a duration of the switching period. Theramp-current generator is further configured to generate the rampcurrent approximately proportional in magnitude to a multiplicationproduct of the duty cycle and a difference, the different representingthe first reference signal minus the compensation signal in magnitude.

In one embodiment, a method for regulating a power conversion systemincludes: generating a drive signal associated with a switching periodincluding an on-time period and an off-time period; and outputting thedrive signal to a switch to affect a current flowing through a primarywinding of a power conversion system. The outputting the drive signal tothe switch to affect the current includes: outputting the drive signalto close the switch during the on-time period; and outputting the drivesignal to open the switch during the off-time period. A duty cycle isequal to a duration of the on-time period divided by a duration of theswitching period. The generating the drive signal associated with theswitching period includes keeping a multiplication product of the dutycycle and the duration of the on-time period approximately constant.

In another embodiment, a method for regulating a power conversion systemincludes: receiving a modulation signal; generating a ramp current basedat least in part on the modulation signal; receiving the ramp current;generating a ramping signal based at least in part on the ramp current;receiving the ramping signal; generating the modulation signal based atleast in part on the ramping signal; receiving the modulation signal;generating a drive signal based at least in part on the modulationsignal, the drive signal being associated with a switching periodincluding an on-time period and an off-time period; and outputting thedrive signal to a switch to affect a current flowing through a primarywinding of a power conversion system. The outputting the drive signal tothe switch to affect the current includes: outputting the drive signalto close the switch during the on-time period; and outputting the drivesignal to open the switch during the off-time period. A duty cycle isequal to a duration of the on-time period divided by a duration of theswitching period. The generating the ramp current based at least in parton the modulation signal includes generating the ramp currentapproximately proportional to the duty cycle in magnitude.

In yet another embodiment, a method for regulating a power conversionsystem includes: providing a compensation signal based on at leastinformation associated with a current flowing through a primary windingof a power conversion system; receiving a modulation signal, thecompensation signal and a first reference signal; generating a rampcurrent based at least in part on the modulation signal, thecompensation signal and the first reference signal; receiving the rampcurrent; generating a ramping signal based at least in part on the rampcurrent; receiving the ramping signal and the compensation signal;generating the modulation signal based at least in part on the rampingsignal and the compensation signal; receiving the modulation signal; andoutputting a drive signal to a switch to affect the current, the drivesignal being associated with a switching period including an on-timeperiod and an off-time period. The outputting the drive signal to theswitch to affect the current includes: outputting the drive signal toclose the switch during the on-time period; outputting the drive signalto open the switch during the off-time period. A duty cycle is equal toa duration of the on-time period divided by a duration of the switchingperiod. The generating the ramp current based at least in part on themodulation signal, the compensation signal and the first referencesignal includes generating the ramp current approximately proportionalin magnitude to a multiplication product of the duty cycle and adifference, the different representing the first reference signal minusthe compensation signal in magnitude.

Depending upon embodiment, one or more benefits may be achieved. Thesebenefits and various additional objects, features and advantages of thepresent invention can be fully appreciated with reference to thedetailed description and accompanying drawings that follow.

4. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram showing a conventional LED lightingsystem.

FIG. 2 is a simplified conventional diagram showing a system controlleras part of the system as shown in FIG. 1.

FIG. 3 is a simplified conventional diagram showing a current-sensingcomponent and an error amplifier as parts of the system controller asshown in FIG. 2.

FIG. 4 is a simplified diagram showing a power conversion systemaccording to an embodiment of the present invention.

FIG. 5(a) is a simplified diagram showing a system controller as part ofthe power conversion system as shown in FIG. 4 according to anembodiment of the present invention.

FIG. 5(b) is a simplified timing diagram for a system controller as partof the power conversion system as shown in FIG. 4 according to anembodiment of the present invention.

FIG. 5(c) is a simplified diagram showing a ramp-current generator aspart of the system controller as shown in FIG. 5(a) according to oneembodiment of the present invention.

FIG. 5(d) is a simplified diagram showing a ramp-current generator and aramp-signal generator as parts of the system controller as shown in FIG.5(a) according to some embodiments of the present invention.

FIG. 6(a) is a simplified diagram showing a system controller as part ofthe power conversion system as shown in FIG. 4 according to anotherembodiment of the present invention.

FIG. 6(b) is a simplified timing diagram for a system controller as partof the power conversion system as shown in FIG. 4 according to anotherembodiment of the present invention.

FIG. 6(c) is a simplified diagram showing a ramp-current generator aspart of the system controller as shown in FIG. 6(a) according to anotherembodiment of the present invention.

FIG. 6(d) is a simplified diagram showing a ramp-current generator and aramp-signal generator as parts of the system controller as shown in FIG.6(a) according to certain embodiments of the present invention.

5. DETAILED DESCRIPTION OF THE INVENTION

Certain embodiments of the present invention are directed to integratedcircuits. More particularly, some embodiments of the invention providesystems and methods for regulating output currents. Merely by way ofexample, some embodiments of the invention have been applied to powerconversion systems. But it would be recognized that the invention has amuch broader range of applicability.

Referring to FIG. 1, to achieve high efficiency (e.g., >90%), the system100 may operate in a quasi-resonant (QR) mode, as an example. A peakvalue of the primary current 162 is determined as follows:

$\begin{matrix}{I_{in\_ peak} = {\left( \frac{T_{on}}{L_{p}} \right) \cdot V_{bulk}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

where I_(in) _(_) _(peak) represents a peak value of the primary current162, T_(on) represents an on-time period during which the power switch128 is closed (e.g., being turned on), V_(bulk) represents the bulkvoltage 150, and L_(p) represents the inductance of the primary winding112.

For example, assuming the on-time period associated with the powerswitch 128 keeps approximately constant for a given input voltage and agiven output load and the inductance of the primary winding 112 keepsapproximately constant, the peak value of the primary current 162follows the bulk voltage 150 (e.g., associated with a rectified sinewaveform), according to Equation 2. In another example, an average ofthe primary current 162 is an average value of the primary current 162during one or more switching periods, or is an average value of theprimary current 162 during one or more switching periods that slide overtime. In yet another example, the average of the primary current 162 isdetermined as follows:

$\begin{matrix}{I_{in\_ ave} = {{\frac{1}{2}{D \cdot I_{in\_ peak}}} = {{\frac{T_{on}}{2\; T_{s}} \cdot I_{in\_ peak}} = {\left( \frac{T_{on}^{2}}{L_{p}} \right) \cdot \frac{V_{bulk}}{2\left( {T_{on} + T_{off}} \right)}}}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

where T_(s) represents a switching period including an on-time period(e.g., T_(on)) during which the power switch 128 is closed (e.g., beingturned on) and an off-time period (e.g., T_(off)) during which the powerswitch 128 is open (e.g., being turned off). In addition, D represents aduty cycle associated with the power switch 128 and is determined asfollows:

$\begin{matrix}{D = \frac{T_{on}}{T_{on} + T_{off}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

If the system 100 operates in the QR mode, the off-time period (e.g.,T_(off)) is the same as a demagnetization period (e.g., T_(demag),associated with a demagnetization process of the secondary winding 114).Assuming the on-time period remains approximately constant in duration,the demagnetization period (e.g., T_(demag)) changes with the peak valueof the primary current 162 and thus the bulk voltage 150. As such, theswitching period (e.g., T_(s)) changes with the bulk voltage 150. If thebulk voltage 150 increases in magnitude, the peak value of the primarycurrent 162 increases and the switch period (e.g., T_(s)) increases induration. As a result, the average of the primary current 162 does notfollow closely the bulk voltage 150 and thus does not have a similarwaveform as the bulk voltage 150 (e.g., a rectified sine waveform),which may result in poor total harmonic distortion.

FIG. 4 is a simplified diagram showing a power conversion systemaccording to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The system 400 includes asystem controller 402, resistors 404, 424, 426, and 432, capacitors 406,420, and 434, a diode 408, a transformer 410 including a primary winding412, a secondary winding 414 and an auxiliary winding 416, a powerswitch 428, a current sensing resistor 430, and a rectifying diode 418.The system controller 402 includes terminals (e.g., pins) 438, 440, 442,444, 446, and 448. For example, the power switch 428 includes a bipolarjunction transistor. In another example, the power switch 428 includes aMOS transistor. In yet another example, the power switch 428 includes aninsulated-gate bipolar transistor (IGBT). The system 400 provides powerto an output load 422, e.g., one or more LEDs. In some embodiments, theresistor 432 is removed. For example, the system 400 operates in aquasi-resonant (QR) mode.

According to some embodiments, the system controller 402 is implementedto vary the duration of an on-time period (e.g., T_(on)) during whichthe power switch 428 keeps closed (e.g., being turned on) with a bulkvoltage 450 which is associated with an alternate-current (AC) inputvoltage 452. For example, the bulk voltage 450 (e.g., a rectifiedvoltage no smaller than 0 V) is received by the resistor 404. In anotherexample, the capacitor 406 is charged in response to the bulk voltage450, and a voltage 454 is provided to the system controller 402 at theterminal 438 (e.g., terminal VCC). In yet another example, if thevoltage 454 is larger than a predetermined threshold voltage inmagnitude, the system controller 402 begins to operate normally, andoutputs a signal 499 through the terminal 442 (e.g., terminal GATE). Inyet another example, the switch 428 is closed (e.g., being turned on) oropen (e.g., being turned off) in response to a drive signal 456associated with the signal 499 so that the output current 458 isregulated to be approximately constant.

According to one embodiment, the auxiliary winding 416 charges thecapacitor 406 through the diode 408 when the switch 428 is opened (e.g.,being turned off) in response to the drive signal 456 so that the systemcontroller 402 can operate normally. For example, a feedback signal 460is provided to the system controller 402 through the terminal 440 (e.g.,terminal FB) in order to detect the end of a demagnetization process ofthe secondary winding 414 for charging or discharging the capacitor 434using an internal error amplifier in the system controller 402. Inanother example, the feedback signal 460 is provided to the systemcontroller 402 through the terminal 440 (e.g., terminal FB) in order todetect the beginning and the end of the demagnetization process of thesecondary winding 414. As an example, the capacitor 434 is charged ordischarged in response to a compensation signal 474 at the terminal 448(e.g., terminal COMP). In another example, the resistor 430 is used fordetecting a primary current 462 flowing through the primary winding 412,and a current-sensing signal 464 is provided to the system controller402 through the terminal 444 (e.g., terminal CS) to be processed duringeach switching cycle (e.g., corresponding to each switching period ofthe power switch 428). In yet another example, peak magnitudes of thecurrent-sensing signal 464 are sampled and provided to the internalerror amplifier. In yet another example, the capacitor 434 is coupled toan output terminal of the internal error amplifier. In yet anotherexample, the capacitor 420 is used to maintain an output voltage 468 soas to keep a stable output current through the output load 422 (e.g.,one or more LEDs). For example, the system 400 implements aprimary-side-regulation scheme with single-stage power factor correction(PFC). As an example, the system 400 implements a flyback architectureor a buck-boost architecture.

According to another embodiment, an average of the primary current 162is an average value of the primary current 162 during one or moreswitching periods, or is an average value of the primary current 162during one or more switching periods that slide over time. For example,the average of the primary current 162 is determined as follows:

$\begin{matrix}{I_{in\_ ave} = {{\frac{1}{2}{D \cdot I_{in\_ peak}}} = {{\frac{1}{2}{D \cdot \frac{T_{on}}{L_{p}} \cdot V_{bulk}}} = {\left( \frac{T_{on}^{2}}{L_{p}} \right) \cdot \frac{V_{bulk}}{2\left( {T_{on} + T_{off}} \right)}}}}} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

where T_(s) represents a switching period including an on-time period(e.g., T_(on)) during which the power switch 428 is closed (e.g., beingturned on) and an off-time period (e.g., T_(off)) during which the powerswitch 428 is open (e.g., being turned off). In another example, a sumof the duration of the on-time period (e.g., T_(on)) and the off-timeperiod (e.g., T_(off)) is equal to the duration of the switching periodT_(s). In addition, D represents a duty cycle associated with the powerswitch 428 and is determined as follows:

$\begin{matrix}{D = \frac{T_{on}}{T_{on} + T_{off}}} & \left( {{Equation}\mspace{14mu} 6} \right)\end{matrix}$

According to certain embodiments, the system controller 402 isimplemented to keep a multiplication product of the duty cycle and theduration of the on-time period constant to achieve low total harmonicdistortion as follows:

D×T _(on)=constant  (Equation 7)

For example, according to Equation 7, if the multiplication product ofthe duty cycle and the duration of the on-time period is kept constant,the average of the primary current 462 changes with the bulk voltage 450(e.g., associated with a rectified sine waveform).

In some embodiments, the system controller 402 is implemented to keep amultiplication product of the duty cycle and the duration of the on-timeperiod approximately constant to achieve low total harmonic distortionas follows:

D×T _(on)≅constant  (Equation 8)

For example, according to Equation 8, if the multiplication product ofthe duty cycle and the duration of the on-time period is keptapproximately constant, the average of the primary current 462 changes(e.g., approximately linearly) with the bulk voltage 450 (e.g.,associated with a rectified sine waveform). In another example, as shownin Equation 8, the error range of the multiplication product of the dutycycle and the duration of the on-time period being constant is ±5%. Inyet another example, as shown in Equation 8, the error range of themultiplication product of the duty cycle and the duration of the on-timeperiod being constant is ±10%. In yet another example, as shown inEquation 8, the error range of the multiplication product of the dutycycle and the duration of the on-time period being constant is ±15%. Inyet another example, as shown in Equation 8, the error range of themultiplication product of the duty cycle and the duration of the on-timeperiod being constant is ±20%.

FIG. 5(a) is a simplified diagram showing the system controller 402 aspart of the power conversion system 400 according to an embodiment ofthe present invention. This diagram is merely an example, which shouldnot unduly limit the scope of the claims. One of ordinary skill in theart would recognize many variations, alternatives, and modifications.The system controller 402 includes a ramp-signal generator 602, anunder-voltage lock-out (UVLO) component 604, a modulation component 606,a logic controller 608, a driving component 610, a demagnetizationdetector 612, an error amplifier 616, a current-sensing-and-sample/holdcomponent 614, a reference-voltage generator 640, and a ramp-currentgenerator 642.

According to one embodiment, the UVLO component 604 detects the signal454 and outputs a signal 618 (e.g., por). For example, if the signal 454is larger than a first predetermined threshold in magnitude, the systemcontroller 402 begins to operate normally. If the signal 454 is smallerthan a second predetermined threshold in magnitude, the systemcontroller 402 is turned off. In another example, the secondpredetermined threshold is smaller than or equal to the firstpredetermined threshold in magnitude. In yet another example, the erroramplifier 616 receives a signal 620 from thecurrent-sensing-and-sample/hold component 614 and a reference signal622. In yet another example, the error amplifier 616 generates a currentwhich charges or discharges the capacitor 434 to generate thecompensation signal 474 (e.g., V_(comp)). In yet another example, thecompensation signal 474 (e.g., V_(comp)) is provided to the modulationcomponent 606. In yet another example, the capacitor 434 is coupled tothe terminal 448 and forms, together with the error amplifier 616, anintegrator or a low-pass filter. In yet another example, the erroramplifier 616 is a transconductance amplifier and outputs a currentwhich is proportional to a difference between the reference signal 622and the signal 620. In yet another example, the error amplifier 616together with the capacitor 434 generates the compensation signal 474(e.g., V_(comp)) which is a voltage signal.

As an example, the reference-voltage generator 640 outputs a referencesignal 636 (e.g., V_(ref1)) to the ramp-current generator 642, outputs avoltage signal 694 (e.g., V1) to the ramp-signal generator 602, andoutputs a reference signal 622 (e.g., V_(ref) _(_) _(ea)) to the erroramplifier 616. In another example, the ramp-signal generator 602 alsoreceives a current signal 638 (e.g., I_(ramp)) generated by theramp-current generator 642 and generates a ramping signal 628. In yetanother example, the current-sensing-and-sample/hold component 614samples the current sensing signal 464 in response to the control signal630 and then holds the sampled signal until thecurrent-sensing-and-sample/hold component 614 samples again the currentsensing signal 464.

According to another embodiment, the current 638 (e.g., I_(ramp)) flowsfrom the ramp-current generator 642 to the ramp-signal generator 602.For example, the current 638 (e.g., I_(ramp)) flows from the ramp-signalgenerator 602 to the ramp-current generator 642. In another example, themodulation component 606 receives the ramping signal 628 and outputs amodulation signal 626. In yet another example, the logic controller 608processes the modulation signal 626 and outputs a control signal 630 tothe current-sensing-and-sample/hold component 614 and the drivingcomponent 610. In yet another example, the modulation signal 626corresponds to a pulse-width-modulation (PWM) signal. In yet anotherexample, the driving component 610 generates the signal 499 related tothe drive signal 456 to affect the switch 428. In yet another example,when the signal 499 is at the logic high level, the signal 456 is at thelogic high level, and when the signal 499 is at the logic low level, thesignal 456 is at the logic low level.

According to yet another embodiment, the demagnetization detector 612detects the feedback signal 460 and outputs a demagnetization signal 632for determining the end of the demagnetization process of the secondarywinding 414. For example, the demagnetization detector 612 detects thefeedback signal 460 and outputs the demagnetization signal 632 fordetermining the beginning and the end of the demagnetization process ofthe secondary winding 414. In another example, the demagnetizationdetector 612 outputs a trigger signal 698 to the logic controller 608 tostart a next cycle (e.g., corresponding to a next switching period).

In one embodiment, the on-time period (e.g., T_(on)) is determined asfollows:

$\begin{matrix}{T_{on} = \frac{V_{comp} - {V\; 1}}{slp}} & \left( {{Equation}\mspace{14mu} 9} \right)\end{matrix}$

where V_(comp) represents the compensation signal 474 (e.g., the outputof the error amplifier 616), V1 represents the signal 694, and slprepresents a slope of the ramping signal 628. For example, the rampingsignal 628 increases, linearly or non-linearly, to a peak magnitudeduring each switching period, and the signal 694 (e.g., V1) correspondsto a start point of the increase of the ramping signal 628. As anexample, the slope of the ramping signal 628 is determined as follows:

$\begin{matrix}{{slp} = \frac{I_{ramp}}{C}} & \left( {{Equation}\mspace{14mu} 10} \right)\end{matrix}$

where I_(ramp) represents the current 638, and C represents thecapacitance of an internal capacitor in the ramp-signal generator 602.Combining Equations 8-10, it is determined:

$\begin{matrix}{{D \times T_{on}} = {{D \times \frac{\left( {V_{comp} - {V\; 1}} \right) \times C}{I_{ramp}}} = {constant}}} & \left( {{Equation}\mspace{14mu} 11} \right)\end{matrix}$

To keep the multiplication product of the duty cycle (e.g., D) and theduration of the on-time period (e.g., T_(on)) constant, the ramp-currentgenerator 642 generates the current 638 (e.g., I_(ramp)) to beproportional in magnitude to the duty cycle (e.g., D), according to someembodiments. For example, the current 638 (e.g., I_(ramp)) is determinedas follows:

I _(ramp) =k ₁ *D  (Equation 12)

where k₁ represents a coefficient parameter (e.g., a constant).

In some embodiments, the ramp-current generator 642 generates thecurrent 638 to be approximately proportional in magnitude to the dutycycle (e.g., D) so that the multiplication product of the duty cycle(e.g., D) and the duration of the on-time period (e.g., T_(on)) is keptapproximately constant. For example, the current 638 (e.g., I_(ramp)) isdetermined as follows:

I _(ramp) ≅k ₁ *D  (Equation 13)

where k₁ represents a coefficient parameter (e.g., a constant). Inanother example, as shown in Equation 13, the error range of the current638 being proportional in magnitude to the duty cycle is ±5%. In yetanother example, as shown in Equation 13, the error range of the current638 being proportional in magnitude to the duty cycle is ±10%. In yetanother example, as shown in Equation 13, the error range of the current638 being proportional in magnitude to the duty cycle is ±15%. In yetanother example, as shown in Equation 13, the error range of the current638 being proportional in magnitude to the duty cycle is ±20%.

As discussed above and further emphasized here, FIG. 5(a) is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, instead of receiving the modulationsignal 626, the ramp-current generator 642 receives the signal 499associated with the drive signal 456. In another example, instead ofreceiving the modulation signal 626, the ramp-current generator 642receives the demagnetization signal 632. In yet another example, insteadof receiving the modulation signal 626, the ramp-current generator 642receives a signal complementary to the demagnetization signal 632.

FIG. 5(b) is a simplified timing diagram for the system controller 402as part of the power conversion system 400 according to an embodiment ofthe present invention. This diagram is merely an example, which shouldnot unduly limit the scope of the claims. One of ordinary skill in theart would recognize many variations, alternatives, and modifications.The waveform 902 represents the modulation signal 626 as a function oftime, the waveform 904 represents the signal 499 as a function of time,the wave form 906 represents the demagnetization signal 632 as afunction of time, the waveform 908 represents the trigger signal 698 asa function of time, and the waveform 910 represents the ramping signal628 as a function of time.

An on-time period and an off-time period associated with the signal 499are shown in FIG. 5(b). The on-time period begins at a time t₃ and endsat a time t₅, and the off-time period begins at the time t₅ and ends ata time t₈. For example, t₀≦t₁≦t₂≦t₃≦t₄≦t₅≦t₆≦t₇≦t₈.

According to one embodiment, at t₀, the demagnetization signal 632changes from the logic high level to the logic low level. For example,the demagnetization detector 612 generates a pulse (e.g., between t₀ andt₂) in the trigger signal 698 to trigger a new cycle. As an example, theramping signal 628 begins to increase from a magnitude 912 to amagnitude 914 (e.g., at t₄). In another example, at t₁, the signal 626changes from the logic low level to the logic high level. After a shortdelay, the signal 499 changes (e.g., at t₃) from the logic low level tothe logic high level, and in response the switch 428 is closed (e.g.,being turned on). In yet another example, at t₄, the signal 626 changesfrom the logic high level to the logic low level, and the ramping signal628 decreases from the magnitude 914 to the magnitude 912. After a shortdelay, the signal 499 changes (e.g., at t₅) from the logic high level tothe logic low level, and in response, the switch 428 is open (e.g.,being turned off). As an example, at t₆, the demagnetization signal 632changes from the logic low level to the logic high level which indicatesa beginning of a demagnetization process. In another example, at t₇, thedemagnetization signal 632 changes from the logic high level to thelogic low level which indicates the end of the demagnetization process.In yet another example, the demagnetization detector 612 generatesanother pulse in the trigger signal 698 to start a next cycle. In yetanother example, the magnitude 912 of the ramping signal 628 isassociated with the signal 694. In yet another example, the magnitude914 of the ramping signal 628 is associated with the magnitude of thecompensation signal 474.

According to another embodiment, the magnitude change of the rampingsignal 628 during the on-time period is determined as follows:

ΔV _(ramp) =V _(comp) −V1=slp×T _(on)  (Equation 14)

where ΔV_(ramp) represents the magnitude changes of the ramping signal628, V_(comp) represents the signal 474, V1 represents the signal 694,slp represents a ramping slope associated with the ramping signal 628,and T_(on) represents the duration of the on-time period. For example,V1 corresponds to the magnitude 912 of the ramping signal 628. Based onEquation 14, the duration of the on-time period is determined asfollows:

$\begin{matrix}{T_{on} = \frac{V_{comp} - {V\; 1}}{slp}} & \left( {{Equation}\mspace{14mu} 15} \right)\end{matrix}$

As shown in Equation 15, for a given compensation signal (e.g., thesignal 474), the duration of the on-time period is determined by theramping slope of the ramping signal 628, according to certainembodiments. For example, a slope of the waveform 910 between t₁ and t₄corresponds to the ramping slope of the ramping signal 628.

FIG. 5(c) is a simplified diagram showing the ramp-current generator 642as part of the system controller 402 according to one embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Theramp-current generator 642 includes an operational amplifier 506, a lowpass filter 508, a voltage-to-current converter 510, a NOT gate 518, andswitches 502 and 504.

According to one embodiment, the switch 502 is closed or opened inresponse to the modulation signal 626 (e.g., PWM), and the switch 504 isclosed or opened in response to a signal 512 (e.g., PWM_b). For example,the NOT gate 518 generates the signal 512 (e.g., PWM_b) which iscomplementary to the modulation signal 626 (e.g., PWM). As an example,if the modulation signal 626 is at the logic high level, the signal 512is at the logic low level, and if the modulation signal 626 is at thelogic low level, the signal 512 is at the logic high level.

In one embodiment, if the modulation signal 626 (e.g., PWM) is at thelogic high level, the switch 502 is closed (e.g., being turned on) andthe operational amplifier 506 receives the reference signal 636 (e.g.,V_(ref1)) at its non-inverting terminal (e.g., terminal “+”), where theinverting terminal (e.g., terminal “−”) and the output terminal of theamplifier 506 are connected. For example, the operational amplifier 506includes a buffer amplifier with a gain of 1. As an example, the signal512 is at the logic low level, and the switch 504 is open (e.g., beingturned off). For example, the low pass filter 508 receives a signal 516from the amplifier 506 and outputs a filtered signal 514 (e.g.,V_(duty)). In another example, the filtered signal 514 (e.g., V_(duty))is a voltage signal and is converted by the voltage-to-current converter510 to the current 638 (e.g., I_(ramp)). In yet another example, thesignal 516 is approximately equal (e.g., in magnitude) to the referencesignal 636.

In another embodiment, if the modulation signal 626 (e.g., PWM) is atthe logic low level and the signal 512 is at the logic high level, theswitch 502 is open (e.g., being turned off), and the switch 504 isclosed (e.g., being turned on). For example, the operational amplifier506 receives a ground voltage 520 at its non-inverting terminal (e.g.,terminal “+”), and changes the signal 516. As an example, the signal 516is approximately equal to the ground voltage 520. As another example,the low pass filter 508 includes a RC filter which includes one or moreresistors and one or more capacitors.

I _(ramp) =k ₁ *D=β(V _(ref1))*D  (Equation 16)

where V_(ref1) represents the reference signal 636, and β represents acoefficient parameter (e.g., a constant).

FIG. 5(d) is a simplified diagram showing the ramp-current generator 642and the ramp-signal generator 602 as parts of the system controller 402according to some embodiments of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The ramp-signal generator602 includes an operational amplifier 546, switches 540 and 542, and acapacitor 544. For example, the switches 502, 504, 540 and 532 eachinclude one or more MOS transistors.

According to one embodiment, the switch 540 is closed or opened inresponse to the modulation signal 626 (e.g., PWM), and the switch 542 isclosed or opened in response to the signal 512 (e.g., PWM_b). In oneembodiment, if the modulation signal 626 (e.g., PWM) is at the logic lowlevel and the signal 512 is at the logic high level, the switch 540 isopen (e.g., being turned off) and the switch 504 is closed (e.g., beingturned on). For example, the operational amplifier 546 receives thesignal 694 (e.g., V1) at its non-inverting terminal (e.g., terminal “+”)and outputs a signal 548, where the inverting terminal (e.g., terminal“−”) and the output terminal of the amplifier 546 are connectedtogether. As an example, the signal 548 is approximately equal (e.g., inmagnitude) to the signal 694 (e.g., V1), and in response the voltage onthe capacitor 544 becomes approximately equal (e.g., in magnitude) tothe signal 548 and thus the signal 694 (e.g., V1).

In another embodiment, if the modulation signal 626 (e.g., PWM) changesto the logic high level and the signal 512 changes to the logic lowlevel, the switch 540 is closed (e.g., being turned on) and the switch504 is opened (e.g., being turned off). For example, the ramp-currentgenerator 642 outputs the current 638 to charge the capacitor 544through the closed switch 540. As an example, the ramping signal 628which corresponds to the voltage on the capacitor 544 increases (e.g.,linearly or non-linearly) from a magnitude approximately equal to thesignal 694 (e.g., V1) to a maximum magnitude (e.g., the compensationsignal 474) as the current 638 charges the capacitor 544.

As discussed above and further emphasized here, FIGS. 5(a), 5(b), 5(c),and 5(d) are merely examples, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. For example, theramp-current generator 642 generates the current 638 (e.g., I_(ramp))based at least in part on a multiplication product of the duty cycle anda difference between the reference signal 636 and the compensationsignal 474 (e.g., V_(comp)), so that the compensation signal 474 (e.g.,V_(comp)) does not vary much at different input voltages to reduce theripple effects of the compensation signal 474, e.g., as shown in FIG.6(a).

FIG. 6(a) is a simplified diagram showing the system controller 402 aspart of the power conversion system 400 according to another embodimentof the present invention. This diagram is merely an example, whichshould not unduly limit the scope of the claims. One of ordinary skillin the art would recognize many variations, alternatives, andmodifications. The system controller 402 includes a ramp-signalgenerator 1602, an under-voltage lock-out (UVLO) component 1604, amodulation component 1606, a logic controller 1608, a driving component1610, a demagnetization detector 1612, an error amplifier 1616, acurrent-sensing-and-sample/hold component 1614, a reference-voltagegenerator 1640, and a ramp-current generator 1642.

For example, the ramp-signal generator 1602, the under-voltage lock-out(UVLO) component 1604, the modulation component 1606, the logiccontroller 1608, the driving component 1610, the demagnetizationdetector 1612, the error amplifier 1616, thecurrent-sensing-and-sample/hold component 1614, the reference-voltagegenerator 1640, and the ramp-current generator 1642 are the same as theramp-signal generator 602, the under-voltage lock-out (UVLO) component604, the modulation component 606, the logic controller 608, the drivingcomponent 610, the demagnetization detector 612, the error amplifier616, the current-sensing-and-sample/hold component 614, thereference-voltage generator 640, and the ramp-current generator 642,respectively.

According to one embodiment, the UVLO component 1604 detects the signal454 and outputs a signal 1618 (e.g., por). For example, if the signal454 is larger than a first predetermined threshold in magnitude, thesystem controller 402 begins to operate normally. If the signal 454 issmaller than a second predetermined threshold in magnitude, the systemcontroller 402 is turned off. In another example, the secondpredetermined threshold is smaller than or equal to the firstpredetermined threshold in magnitude. In yet another example, the erroramplifier 1616 receives a signal 1620 from thecurrent-sensing-and-sample/hold component 1614 and a reference signal1622, and the compensation signal 474 (e.g., V_(comp)) is provided tothe modulation component 1606 and the voltage-to-current-conversioncomponent 1642. In yet another example, the capacitor 434 is coupled tothe terminal 448 and forms, together with the error amplifier 1616, anintegrator or a low-pass filter. In yet another example, the erroramplifier 1616 is a transconductance amplifier and outputs a currentwhich is proportional to a difference between the reference signal 1622and the signal 1620. In yet another example, the error amplifier 1616together with the capacitor 434 generates the compensation signal 474(e.g., V_(comp)) which is a voltage signal.

As an example, the reference-voltage generator 1640 outputs a referencesignal 1636 (e.g., V_(ref)) to the ramp-current generator 1642, outputsa voltage signal 1694 (e.g., V1) to the ramp-signal generator 1602, andoutputs a reference signal 1622 (e.g., V_(ref) _(_) _(ea)) to the erroramplifier 1616. In another example, the ramp-signal generator 1602 alsoreceives a current signal 1638 (e.g., I_(ramp)) generated by theramp-current generator 1642 and generates a ramping signal 1628. In oneembodiment, the current signal 1638 is equal in magnitude to the currentsignal 638. In another embodiment, the current signal 1638 is not equalin magnitude to the current signal 638.

According to another embodiment, the current 1638 (e.g., I_(ramp)) flowsfrom the ramp-current generator 1642 to the ramp-signal generator 1602.For example, the current 1638 (e.g., I_(ramp)) flows from theramp-signal generator 1602 to the ramp-current generator 1642. Inanother example, the modulation component 1606 receives the rampingsignal 1628 and outputs a modulation signal 1626. In yet anotherexample, the logic controller 1608 processes the modulation signal 1626and outputs a control signal 1630 to the current-sensing-and-sample/holdcomponent 1614 and the driving component 1610. In yet another example,the modulation signal 1626 corresponds to a pulse-width-modulation (PWM)signal.

According to yet another embodiment, the current-sensing-and-sample/holdcomponent 1614 samples the current sensing signal 464 in response to thecontrol signal 1630 and then holds the sampled signal until thecurrent-sensing-and-sample/hold component 1614 samples again the currentsensing signal 464. For example, the driving component 1610 generatesthe signal 499 related to the drive signal 456 to affect the switch 428.In another example, if the signal 499 is at the logic high level, thesignal 456 is at the logic high level, and if the signal 499 is at thelogic low level, the signal 456 is at the logic low level. As anexample, the demagnetization detector 1612 detects the feedback signal460 and outputs a demagnetization signal 1632 for determining the end ofthe demagnetization process of the secondary winding 414. As anotherexample, the demagnetization detector 1612 detects the feedback signal460 and outputs the demagnetization signal 1632 for determining thebeginning and the end of the demagnetization process of the secondarywinding 414. In yet another example, the demagnetization detector 1612outputs a trigger signal 1698 to the logic controller 1608 to start anext cycle (e.g., corresponding to a next switching period).

To keep the multiplication product of the duty cycle (e.g., D) and theduration of the on-time period (e.g., T_(on)) constant, the ramp-currentgenerator 1642 generates the current 1638 (e.g., I_(ramp)) to beproportional in magnitude to the duty cycle (e.g., D), according to someembodiments. For example, the current 1638 (e.g., I_(ramp)) isdetermined as follows:

I _(ramp) =k _(2*D)  (Equation 17)

where k₂ represents a coefficient parameter. As an example, k₂ isproportional to a difference between the reference signal 1636 (e.g.,V_(ref)) and the compensation signal 474 (e.g., V_(comp)). For example,a differential signal is generated based at least in part on thedifference between the reference signal 1636 (e.g., V_(ref)) and thecompensation signal 474 (e.g., V_(comp)). In certain embodiments, thecurrent 1638 (e.g., I_(ramp)) is determined as follows:

I _(ramp)=α(V _(ref) −V _(comp))×D  (Equation 18)

where α represents a coefficient parameter (e.g., a constant). In someapplications, the compensation signal 474 (e.g., V_(comp)), e.g., theoutput of the error amplifier 1616, represents an output load conditionfor a given input voltage (e.g., V_(bulk)), according to certainembodiments.

In some embodiments, the ramp-current generator 1642 generates thecurrent 1638 to be approximately proportional in magnitude to the dutycycle (e.g., D) so that the multiplication product of the duty cycle(e.g., D) and the duration of the on-time period (e.g., T_(on)) is keptapproximately constant. For example, the current 1638 (e.g., I_(ramp))is determined as follows:

I _(ramp) ≅k ₂ *D  (Equation 19)

where k₂ represents a coefficient parameter. As an example, k₂ isapproximately proportional to a difference between the reference signal1636 (e.g., V_(ref)) and the compensation signal 474 (e.g., V_(comp)).For example, a differential signal is generated based at least in parton the difference between the reference signal 1636 (e.g., V_(ref)) andthe compensation signal 474 (e.g., V_(comp)). In certain embodiments,the current 1638 (e.g., I_(ramp)) is determined as follows:

I _(ramp)≅α(V _(ref) −V _(comp))×D  (Equation 20)

where a represents a coefficient parameter (e.g., a constant). Forexample, as shown in Equation 20, the error range of the current 1638being proportional in magnitude to a multiplication product of the dutycycle and the difference between the reference signal 1636 and thecompensation signal 474 is ±5%. In another example, as shown in Equation20, the error range of the current 1638 being proportional in magnitudeto a multiplication product of the duty cycle and the difference betweenthe reference signal 1636 and the compensation signal 474 is ±10%. Inyet another example, as shown in Equation 20, the error range of thecurrent 1638 being proportional in magnitude to a multiplication productof the duty cycle and the difference between the reference signal 1636and the compensation signal 474 is ±15%. In yet another example, asshown in Equation 20, the error range of the current 1638 beingproportional in magnitude to a multiplication product of the duty cycleand the difference between the reference signal 1636 and thecompensation signal 474 is ±20%.

As discussed above and further emphasized here, FIG. 6(a) is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, instead of receiving the modulationsignal 1626, the ramp-current generator 1642 receives the signal 499associated with the drive signal 456. In another example, instead ofreceiving the modulation signal 1626, the ramp-current generator 1642receives the demagnetization signal 1632. In yet another example,instead of receiving the modulation signal 1626, the ramp-currentgenerator 1642 receives a signal complementary to the demagnetizationsignal 1632.

FIG. 6(b) is a simplified timing diagram for the system controller 402as part of the power conversion system 400 according to anotherembodiment of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. The waveform 1902 represents the modulation signal 1626as a function of time, the waveform 1904 represents the signal 499 as afunction of time, the wave form 1906 represents the demagnetizationsignal 1632 as a function of time, the waveform 1908 represents thetrigger signal 1698 as a function of time, and the waveform 1910represents the ramping signal 1628 as a function of time.

An on-time period and an off-time period associated with the signal 499are shown in FIG. 6(b). The on-time period begins at a time t₁₃ and endsat a time t₁₅, and the off-time period begins at the time t₁₅ and endsat a time t₁₈. For example, t₁₀≦t₁₁≦t₁₂≦t₁₃≦t₁₄≦t₁₅≦t₁₆≦t₁₇≦t₁₈.

According to one embodiment, at t₁₀, the demagnetization signal 1632changes from the logic high level to the logic low level. For example,the demagnetization detector 1612 generates a pulse (e.g., between t₁₀and t₁₂) in the trigger signal 1698 to trigger a new cycle. As anexample, the ramping signal 1628 begins to increase from a magnitude1912 to a magnitude 1914 (e.g., at t₁₄). In another example, at t₁₁, thesignal 1626 changes from the logic low level to the logic high level.After a short delay, the signal 499 changes (e.g., at t₁₃) from thelogic low level to the logic high level, and in response the switch 428is closed (e.g., being turned on). In yet another example, at t₁₄, thesignal 1626 changes from the logic high level to the logic low level,and the ramping signal 1628 decreases from the magnitude 1914 to themagnitude 1912. After a short delay, the signal 499 changes (e.g., att₁₅) from the logic high level to the logic low level, and in response,the switch 428 is open (e.g., being turned off). As an example, at t₁₆,the demagnetization signal 1632 changes from the logic low level to thelogic high level which indicates a beginning of a demagnetizationprocess. In another example, at t₁₇, the demagnetization signal 1632changes from the logic high level to the logic low level which indicatesthe end of the demagnetization process. In yet another example, thedemagnetization detector 1612 generates another pulse in the triggersignal 1698 to start a next cycle. In yet another example, the magnitude1912 of the ramping signal 1628 is associated with the signal 1694. Inyet another example, the magnitude 1914 of the ramping signal 1628 isassociated with the magnitude of the compensation signal 474. In yetanother example, a ramping slope of the ramp signal 1628 is modulated bythe compensation signal 474 (e.g., V_(comp)), e.g., the output of theerror amplifier 1616.

According to another embodiment, the magnitude change of the rampingsignal 1628 during the on-time period is determined as follows:

ΔV _(ramp) =V _(comp) −V1=slp×T _(on)  (Equation 21)

where ΔV_(ramp) represents the magnitude changes of the ramping signal1628, V_(comp) represents the signal 474, V1 represents the signal 1694,slp represents a ramping slope associated with the ramping signal 1628,and T_(on) represents the duration of the on-time period. For example,V1 corresponds to the magnitude 1912 of the ramping signal 1628. Basedon Equation 15, the duration of the on-time period is determined asfollows:

$\begin{matrix}{T_{on} = \frac{V_{comp} - {V\; 1}}{slp}} & \left( {{Equation}\mspace{14mu} 22} \right)\end{matrix}$

As shown in Equation 22, for a given compensation signal (e.g., theoutput of the error amplifier 1616), the duration of the on-time periodis determined by the ramping slope of the ramping signal 1628, accordingto certain embodiments. For example, a slope of the waveform 1910between t₁₁ and t₁₄ corresponds to the ramping slope of the rampingsignal 1628. In some embodiments, the ramping slope of the rampingsignal 1628 is the same as the ramping slope of the ramping signal 628.In certain embodiments, the ramping slope of the ramping signal 1628 isdifferent from the ramping slope of the ramping signal 628.

FIG. 6(c) is a simplified diagram showing the ramp-current generator1642 as part of the system controller 402 according to anotherembodiment of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. The ramp-current generator 1642 includes an operationalamplifier 1506, a low pass filter 1508, a voltage-to-current converter1510, a NOT gate 1518, a summation component 1522, and switches 1502 and1504. For example, the operational amplifier 1506, the low pass filter1508, the voltage-to-current converter 1510, the NOT gate 1518, and theswitches 1502 and 1504 are the same as the operational amplifier 506,the low pass filter 508, the voltage-to-current converter 510, the NOTgate 518, and the switches 502 and 504, respectively.

According to one embodiment, the switch 1502 is closed or opened inresponse to the modulation signal 1626 (e.g., PWM), and the switch 1504is closed or opened in response to a signal 1512 (e.g., PWM_b). Forexample, the NOT gate 1518 generates the signal 1512 (e.g., PWM_b) whichis complementary to the modulation signal 1626 (e.g., PWM). As anexample, if the modulation signal 1626 is at the logic high level, thesignal 1512 is at the logic low level, and if the modulation signal 1626is at the logic low level, the signal 1512 is at the logic high level.In another example, the summation component 1522 receives the referencesignal 1636 (e.g., V_(ref)) and the compensation signal 474 (e.g.,V_(comp)) and generates a signal 1524, where the signal 1524 is equal(e.g., in magnitude) to a difference between the reference signal 1636(e.g., V_(ref)) and the compensation signal 474 (e.g., V_(comp)).

In one embodiment, if the modulation signal 1626 (e.g., PWM) is at thelogic high level, the switch 1502 is closed (e.g., being turned on) andthe operational amplifier 1506 receives the signal 1524 at itsnon-inverting terminal (e.g., terminal “+”), where the invertingterminal (e.g., terminal “−”) and the output terminal of the amplifier1506 are connected together. As an example, the signal 1512 is at thelogic low level, and the switch 1504 is open (e.g., being turned off).For example, the low pass filter 1508 receives a signal 1516 from theamplifier 1506 and outputs a filtered signal 1514 (e.g., V_(duty)). Inanother example, the filtered signal 1514 (e.g., V_(duty)) is a voltagesignal and is converted by the voltage-to-current converter 1510 to thecurrent 1638 (e.g., I_(ramp)). In yet another example, the signal 1516is approximately equal (e.g., in magnitude) to the signal 1524.

In another embodiment, if the modulation signal 1626 (e.g., PWM) is atthe logic low level and the signal 1512 is at the logic high level, theswitch 1502 is open (e.g., being turned off), and the switch 1504 isclosed (e.g., being turned on). For example, the operational amplifier1506 receives a ground voltage 1520 at its non-inverting terminal (e.g.,terminal “+”), and changes the signal 1516. As an example, the signal1516 is approximately equal to the ground voltage 1520. As anotherexample, the low pass filter 1508 includes a RC filter which includesone or more resistors and one or more capacitors.

FIG. 6(d) is a simplified diagram showing the ramp-current generator1642 and the ramp-signal generator 1602 as parts of the systemcontroller 402 according to certain embodiments of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. Theramp-signal generator 1602 includes an operational amplifier 1546,switches 1540 and 1542, and a capacitor 1544. For example, the switches1502, 1504, 1540 and 1542 each include one or more MOS transistors.

According to one embodiment, the switch 1540 is closed or opened inresponse to the modulation signal 1626 (e.g., PWM), and the switch 1542is closed or opened in response to the signal 1512 (e.g., PWM_b). In oneembodiment, if the modulation signal 1626 (e.g., PWM) is at the logiclow level and the signal 1512 is at the logic high level, the switch1540 is open (e.g., being turned off) and the switch 1504 is closed(e.g., being turned on). For example, the operational amplifier 1546receives the signal 1694 (e.g., V1) at its non-inverting terminal (e.g.,terminal “+”) and outputs a signal 1548, where the inverting terminal(e.g., terminal “−”) and the output terminal of the amplifier 1546 areconnected together. In another example, the operational amplifier 1546includes a buffer amplifier with a gain of 1. As an example, the signal1548 is approximately equal (e.g., in magnitude) to the signal 1694(e.g., V1), and in response the voltage on the capacitor 1544 becomesapproximately equal (e.g., in magnitude) to the signal 1548 and thus thesignal 1694 (e.g., V1).

In another embodiment, if the modulation signal 1626 (e.g., PWM) changesto the logic high level and the signal 1512 changes to the logic lowlevel, the switch 1540 is closed (e.g., being turned on) and the switch1504 is opened (e.g., being turned off). For example, the ramp-currentgenerator 1642 outputs the current 1638 to charge the capacitor 1544through the closed switch 1540. As an example, the ramping signal 1628which corresponds to the voltage on the capacitor 1544 increases (e.g.,linearly or non-linearly) from a magnitude approximately equal to thesignal 1694 (e.g., V1) to a maximum magnitude (e.g., the compensationsignal 474) as the current 1638 charges the capacitor 1544.

According to one embodiment, a system controller for regulating a powerconversion system includes: a first controller terminal configured toreceive a first signal related to an input signal for a primary windingof a power conversation system; and a second controller terminalconfigured to output a drive signal to a switch to affect a currentflowing through the primary winding of the power conversion system, thedrive signal being associated with a switching period including anon-time period and an off-time period. The switch is closed in responseto the drive signal during the on-time period. The switch is opened inresponse to the drive signal during the off-time period. A duty cycle isequal to a duration of the on-time period divided by a duration of theswitching period. The system controller is configured to keep amultiplication product of the duty cycle and the duration of the on-timeperiod approximately constant. For example, the system controller isimplemented according to at least FIG. 4, FIG. 5(a), FIG. 5(b), FIG.5(c), and/or FIG. 5(d).

According to another embodiment, a system controller for regulating apower conversion system includes: a ramp-current generator configured toreceive a modulation signal and generate a ramp current based at leastin part on the modulation signal; a ramp-signal generator configured toreceive the ramp current and generate a ramping signal based at least inpart on the ramp current; a modulation component configured to receivethe ramping signal and generate the modulation signal based at least inpart on the ramping signal; a driving component configured to receivethe modulation signal and output a drive signal to a switch to affect acurrent flowing through a primary winding of a power conversion system,the drive signal being associated with a switching period including anon-time period and an off-time period. The switch is closed in responseto the drive signal during the on-time period. The switch is opened inresponse to the drive signal during the off-time period. A duty cycle isequal to a duration of the on-time period divided by a duration of theswitching period. The ramp-current generator is further configured togenerate the ramp current approximately proportional to the duty cyclein magnitude. For example, the system controller is implementedaccording to at least FIG. 4, FIG. 5(a), FIG. 5(b), FIG. 5(c), and/orFIG. 5(d).

According to yet another embodiment, a system controller for regulatinga power conversion system includes: a first controller terminalconfigured to provide a compensation signal based on at leastinformation associated with a current flowing through a primary windingof a power conversion system; a ramp-current generator configured toreceive a modulation signal, the compensation signal and a firstreference signal and generate a ramp current based at least in part onthe modulation signal, the compensation signal and the first referencesignal; a ramp-signal generator configured to receive the ramp currentand generate a ramping signal based at least in part on the rampcurrent; a modulation component configured to receive the ramping signaland the compensation signal and generate the modulation signal based atleast in part on the ramping signal and the compensation signal; and adriving component configured to receive the modulation signal and outputa drive signal to a switch to affect the current, the drive signal beingassociated with a switching period including an on-time period and anoff-time period. The switch is closed in response to the drive signalduring the on-time period. The switch is opened in response to the drivesignal during the off-time period. A duty cycle is equal to a durationof the on-time period divided by a duration of the switching period. Theramp-current generator is further configured to generate the rampcurrent approximately proportional in magnitude to a multiplicationproduct of the duty cycle and a difference, the different representingthe first reference signal minus the compensation signal in magnitude.For example, the system controller is implemented according to at leastFIG. 4, FIG. 6(a), FIG. 6(b), FIG. 6(c), and/or FIG. 6(d).

In one embodiment, a method for regulating a power conversion systemincludes: generating a drive signal associated with a switching periodincluding an on-time period and an off-time period; and outputting thedrive signal to a switch to affect a current flowing through a primarywinding of a power conversion system. The outputting the drive signal tothe switch to affect the current includes: outputting the drive signalto close the switch during the on-time period; and outputting the drivesignal to open the switch during the off-time period. A duty cycle isequal to a duration of the on-time period divided by a duration of theswitching period. The generating the drive signal associated with theswitching period includes keeping a multiplication product of the dutycycle and the duration of the on-time period approximately constant. Forexample, the method is implemented according to at least FIG. 4, FIG.5(a), FIG. 5(b), FIG. 5(c), and/or FIG. 5(d).

In another embodiment, a method for regulating a power conversion systemincludes: receiving a modulation signal; generating a ramp current basedat least in part on the modulation signal; receiving the ramp current;generating a ramping signal based at least in part on the ramp current;receiving the ramping signal; generating the modulation signal based atleast in part on the ramping signal; receiving the modulation signal;generating a drive signal based at least in part on the modulationsignal, the drive signal being associated with a switching periodincluding an on-time period and an off-time period; and outputting thedrive signal to a switch to affect a current flowing through a primarywinding of a power conversion system. The outputting the drive signal tothe switch to affect the current includes: outputting the drive signalto close the switch during the on-time period; and outputting the drivesignal to open the switch during the off-time period. A duty cycle isequal to a duration of the on-time period divided by a duration of theswitching period. The generating the ramp current based at least in parton the modulation signal includes generating the ramp currentapproximately proportional to the duty cycle in magnitude. For example,the method is implemented according to at least FIG. 4, FIG. 5(a), FIG.5(b), FIG. 5(c), and/or FIG. 5(d).

In yet another embodiment, a method for regulating a power conversionsystem includes: providing a compensation signal based on at leastinformation associated with a current flowing through a primary windingof a power conversion system; receiving a modulation signal, thecompensation signal and a first reference signal; generating a rampcurrent based at least in part on the modulation signal, thecompensation signal and the first reference signal; receiving the rampcurrent; generating a ramping signal based at least in part on the rampcurrent; receiving the ramping signal and the compensation signal;generating the modulation signal based at least in part on the rampingsignal and the compensation signal; receiving the modulation signal; andoutputting a drive signal to a switch to affect the current, the drivesignal being associated with a switching period including an on-timeperiod and an off-time period. The outputting the drive signal to theswitch to affect the current includes: outputting the drive signal toclose the switch during the on-time period; outputting the drive signalto open the switch during the off-time period. A duty cycle is equal toa duration of the on-time period divided by a duration of the switchingperiod. The generating the ramp current based at least in part on themodulation signal, the compensation signal and the first referencesignal includes generating the ramp current approximately proportionalin magnitude to a multiplication product of the duty cycle and adifference, the different representing the first reference signal minusthe compensation signal in magnitude. For example, the method isimplemented according to at least FIG. 4, FIG. 6(a), FIG. 6(b), FIG.6(c), and/or FIG. 6(d).

For example, some or all components of various embodiments of thepresent invention each are, individually and/or in combination with atleast another component, implemented using one or more softwarecomponents, one or more hardware components, and/or one or morecombinations of software and hardware components. In another example,some or all components of various embodiments of the present inventioneach are, individually and/or in combination with at least anothercomponent, implemented in one or more circuits, such as one or moreanalog circuits and/or one or more digital circuits. In yet anotherexample, various embodiments and/or examples of the present inventioncan be combined.

Although specific embodiments of the present invention have beendescribed, it will be understood by those of skill in the art that thereare other embodiments that are equivalent to the described embodiments.Accordingly, it is to be understood that the invention is not to belimited by the specific illustrated embodiments, but only by the scopeof the appended claims.

What is claimed is:
 1. A system controller for regulating a powerconversion system, the system controller comprising: a first controllerterminal configured to receive a first signal related to an input signalfor a primary winding of a power conversation system; and a secondcontroller terminal configured to output a drive signal to a switch toaffect a current flowing through the primary winding of the powerconversion system, the drive signal being associated with a switchingperiod including an on-time period and an off-time period; wherein: theswitch is closed in response to the drive signal during the on-timeperiod; the switch is opened in response to the drive signal during theoff-time period; and a duty cycle is equal to a duration of the on-timeperiod divided by a duration of the switching period; wherein the systemcontroller is configured to keep a multiplication product of the dutycycle and the duration of the on-time period approximately constant. 2.The system controller of claim 1 wherein the system controller isfurther configured to keep a multiplication product of the duty cycleand the duration of the on-time period constant.
 3. The systemcontroller of claim 1 wherein the system controller is configured tokeep a multiplication product of the duty cycle and the duration of theon-time period approximately constant so that an average of the currentduring one or more switching periods increases in magnitude with theincreasing input signal over time and decreases in magnitude with thedecreasing input signal over time.
 4. The system controller of claim 3wherein: the input signal changes over time along a first waveform; andthe average of the current changes over time along a second waveformsimilar to the first waveform.
 5. The system controller of claim 1,further comprising: a ramp-current generator configured to receive amodulation signal and generate a ramp current based at least in part onthe modulation signal; a ramp-signal generator configured to receive theramp current and generate a ramping signal based at least in part on theramp current; a modulation component configured to receive the rampingsignal and generate the modulation signal based at least in part on theramping signal; and a driving component configured to receive themodulation signal and output the drive signal.
 6. The system controllerof claim 5 wherein the ramp-current generator includes: an amplifierconfigured to generate an amplified signal based at least in part on themodulation signal; a low-pass filter configured to receive the amplifiedsignal and generate a filtered signal based at least in part on theamplified signal; and a voltage-to-current converter configured toreceive the filtered signal and generate the ramp current based at leastin part on the filtered signal.
 7. The system controller of claim 6wherein the amplifier is further configured to: in response to themodulation signal being at a first logic level, receive a referencesignal and generate the amplified signal based at least in part on thereference signal; and in response to the modulation signal being at asecond logic level, receive a ground voltage and generate the amplifiedsignal based at least in part on the ground voltage.
 8. The systemcontroller of claim 7 wherein the ramp-current generator furtherincludes: a first switch coupled to the amplifier and configured to beclosed to pass the reference signal in response to the modulation signalbeing at the first logic level; and a second switch coupled to theamplifier and configured to be closed to pass the ground voltage inresponse to the modulation signal being at the second logic level. 9.The system controller of claim 5 wherein the ramp-signal generatorincludes: a capacitor configured to: in response to the modulationsignal being at a first logic level, generate the ramping signal basedat least in part on the ramp current; and in response to the modulationsignal being at a second logic level, generate the ramping signal basedat least in part on a second signal.
 10. The system controller of claim9 wherein the ramp-signal generator further includes: an amplifierconfigured to receive a reference signal and generate the second signalbased at least in part on the reference signal.
 11. The systemcontroller of claim 9, further comprising: a first switch coupled to thecapacitor and configured to be closed to pass the ramp current inresponse to the modulation signal being at the first logic level; and asecond switch coupled to the capacitor and configured to be closed topass the second signal in response to the modulation signal being at thesecond logic level.
 12. The system controller of claim 5, furthercomprising: a demagnetization detector configured to receive a feedbacksignal associated with an output signal of the power conversion systemand generate a trigger signal based at least in part on the feedbacksignal, the trigger signal indicating an end of a demagnetizationprocess of the power conversion system; and a logic controllerconfigured to receive the trigger signal and generate a second signalbased at least in part on the trigger signal to affect the drive signal.13. The system controller of claim 12, further comprising: an erroramplifier configured to receive a reference signal and generate a thirdsignal based at least in part on the reference signal.
 14. The systemcontroller of claim 13, further comprising: a current sensing componentconfigured to receive a current sensing signal associated with thecurrent and generate a fourth signal based at least in part on thecurrent sensing signal; wherein: the demagnetization detector is furtherconfigured to generate a demagnetization signal associated with thedemagnetization process based at least in part on the feedback signal;and the error amplifier is further configured to: in response to thedemagnetization signal being at a first logic level, receive the fourthsignal and generate the third signal indicating a difference between thefourth signal and the reference signal; and in response to thedemagnetization signal being at a second logic level, receive a groundvoltage and generate the third signal indicating a difference betweenthe fourth signal and the ground voltage.
 15. The system controller ofclaim 13 wherein: the error amplifier is further configured to generatethe third signal to charge or discharge a capacitor to generate acompensation signal; and the modulation component is further configuredto receive the compensation signal and generate the modulation signalbased at least in part on the compensation signal and the rampingsignal.
 16. The system controller of claim 5 wherein the ramping signalincreases in magnitude at a ramping slope during the on-time period. 17.A system controller for regulating a power conversion system, the systemcontroller comprising: a ramp-current generator configured to receive amodulation signal and generate a ramp current based at least in part onthe modulation signal; a ramp-signal generator configured to receive theramp current and generate a ramping signal based at least in part on theramp current; a modulation component configured to receive the rampingsignal and generate the modulation signal based at least in part on theramping signal; and a driving component configured to receive themodulation signal and output a drive signal to a switch to affect acurrent flowing through a primary winding of a power conversion system,the drive signal being associated with a switching period including anon-time period and an off-time period; wherein: the switch is closed inresponse to the drive signal during the on-time period; the switch isopened in response to the drive signal during the off-time period; and aduty cycle is equal to a duration of the on-time period divided by aduration of the switching period; wherein the ramp-current generator isfurther configured to generate the ramp current approximatelyproportional to the duty cycle in magnitude.
 18. The system controllerof claim 17 is configured to keep a multiplication product of the dutycycle and the duration of the on-time period approximately constant inresponse to the ramp current being approximately proportional to theduty cycle in magnitude.
 19. The system controller of claim 17 whereinthe ramp-current generator is further configured to generate the rampcurrent proportional to the duty cycle in magnitude.
 20. The systemcontroller of claim 19 is configured to keep a multiplication product ofthe duty cycle and the duration of the on-time period constant inresponse to the ramp current being proportional to the duty cycle inmagnitude.
 21. The system controller of claim 17 wherein theramp-current generator includes: an amplifier configured to generate anamplified signal based at least in part on the modulation signal; alow-pass filter configured to receive the amplified signal and generatea filtered signal based at least in part on the amplified signal; and avoltage-to-current converter configured to receive the filtered signaland generate the ramp current based at least in part on the filteredsignal.
 22. The system controller of claim 21 wherein the amplifier isfurther configured to: in response to the modulation signal being at afirst logic level, receive a reference signal and generate the amplifiedsignal based at least in part on the reference signal; and in responseto the modulation signal being at a second logic level, receive a groundvoltage and generate the amplified signal based at least in part on theground voltage.
 23. The system controller of claim 22 wherein theramp-current generator further includes: a first switch coupled to theamplifier and configured to be closed to pass the reference signal inresponse to the modulation signal being at the first logic level; and asecond switch coupled to the amplifier and configured to be closed topass the ground voltage in response to the modulation signal being atthe second logic level.
 24. The system controller of claim 17 whereinthe ramp-signal generator includes: a capacitor configured to: inresponse to the modulation signal being at a first logic level, generatethe ramping signal based at least in part on the ramp current; and inresponse to the modulation signal being at a second logic level,generate the ramping signal based at least in part on a second signal.25. The system controller of claim 24 wherein the ramp-signal generatorfurther includes: an amplifier configured to receive a reference signaland generate the second signal based at least in part on the referencesignal.
 26. The system controller of claim 24, further comprising: afirst switch coupled to the capacitor and configured to be closed topass the ramp current in response to the modulation signal being at thefirst logic level; and a second switch coupled to the capacitor andconfigured to be closed to pass the second signal in response to themodulation signal being at the second logic level.
 27. The systemcontroller of claim 17, further comprising: a demagnetization detectorconfigured to receive a feedback signal associated with an output signalof the power conversion system and generate a trigger signal based atleast in part on the feedback signal, the trigger signal indicating anend of a demagnetization process of the power conversion system; and alogic controller configured to receive the trigger signal and generate asecond signal based at least in part on the trigger signal to affect thedrive signal.
 28. The system controller of claim 27, further comprising:an error amplifier configured to receive a reference signal and generatea third signal based at least in part on the reference signal.
 29. Thesystem controller of claim 28, further comprising: a current sensingcomponent configured to receive a current sensing signal associated withthe current and generate a fourth signal based at least in part on thecurrent sensing signal; wherein: the demagnetization detector is furtherconfigured to generate a demagnetization signal associated with thedemagnetization process based at least in part on the feedback signal;and the error amplifier is further configured to: in response to thedemagnetization signal being at a first logic level, receive the fourthsignal and generate the third signal indicating a difference between thefourth signal and the reference signal; and in response to thedemagnetization signal being at a second logic level, receive a groundvoltage and generate the third signal indicating a difference betweenthe fourth signal and the ground voltage.
 30. The system controller ofclaim 28 wherein: the error amplifier is further configured to generatethe third signal to charge a capacitor to generate a compensationsignal; and the modulation component is further configured to receivethe compensation signal and generate the modulation signal based atleast in part on the compensation signal and the ramping signal.
 31. Thesystem controller of claim 17 wherein the ramping signal increases inmagnitude at a ramping slope during the on-time period.
 32. A systemcontroller for regulating a power conversion system, the systemcontroller comprising: a first controller terminal configured to providea compensation signal based on at least information associated with acurrent flowing through a primary winding of a power conversion system;a ramp-current generator configured to receive a modulation signal, thecompensation signal and a first reference signal and generate a rampcurrent based at least in part on the modulation signal, thecompensation signal and the first reference signal; a ramp-signalgenerator configured to receive the ramp current and generate a rampingsignal based at least in part on the ramp current; a modulationcomponent configured to receive the ramping signal and the compensationsignal and generate the modulation signal based at least in part on theramping signal and the compensation signal; and a driving componentconfigured to receive the modulation signal and output a drive signal toa switch to affect the current, the drive signal being associated with aswitching period including an on-time period and an off-time period;wherein: the switch is closed in response to the drive signal during theon-time period; the switch is opened in response to the drive signalduring the off-time period; and a duty cycle is equal to a duration ofthe on-time period divided by a duration of the switching period;wherein the ramp-current generator is further configured to generate theramp current approximately proportional in magnitude to a multiplicationproduct of the duty cycle and a difference, the different representingthe first reference signal minus the compensation signal in magnitude.33. The system controller of claim 32 is configured to keep amultiplication product of the duty cycle and the duration of the on-timeperiod approximately constant in response to the ramp current beingapproximately proportional in magnitude to the multiplication product ofthe duty cycle and the difference.
 34. The system controller of claim 32wherein the ramp-current generator is further configured to generate theramp current proportional in magnitude to the multiplication product ofthe duty cycle and the difference.
 35. The system controller of claim 34is configured to keep a multiplication product of the duty cycle and theduration of the on-time period constant in response to the ramp currentbeing proportional in magnitude to the multiplication product of theduty cycle and the difference.
 36. The system controller of claim 32wherein the ramp-current generator includes: an amplifier configured togenerate an amplified signal based at least in part on the modulationsignal; a low-pass filter configured to receive the amplified signal andgenerate a filtered signal based at least in part on the amplifiedsignal; and a voltage-to-current converter configured to receive thefiltered signal and generate the ramp current based at least in part onthe filtered signal.
 37. The system controller of claim 36 wherein theamplifier is further configured to: in response to the modulation signalbeing at a first logic level, receive a differential signal indicating adifference between the first reference signal and the compensationsignal and generate the amplified signal based at least in part on thedifferential signal; and in response to the modulation signal being at asecond logic level, receive a ground voltage and generate the amplifiedsignal based at least in part on the ground voltage.
 38. The systemcontroller of claim 37 wherein the ramp-current generator furtherincludes: a first switch coupled to the amplifier and configured to beclosed to pass the differential signal in response to the modulationsignal being at the first logic level; and a second switch coupled tothe amplifier and configured to be closed to pass the ground voltage inresponse to the modulation signal being at the second logic level. 39.The system controller of claim 37 wherein the ramp-current generatorfurther includes: a summation component configured to receive the firstreference signal and the compensation signal and generate thedifferential signal based at least in part on the first reference signaland the compensation signal.
 40. The system controller of claim 37,further comprising: an error amplifier configured to receive a secondreference signal and generate a second signal based at least in part onthe second reference signal to charge a capacitor to generate thecompensation signal.
 41. The system controller of claim 32 wherein theramp-signal generator includes: a capacitor configured to: in responseto the modulation signal being at a first logic level, generate theramping signal based at least in part on the ramp current; and inresponse to the modulation signal being at a second logic level,generate the ramping signal based at least in part on a second signal.42. The system controller of claim 41 wherein the ramp-signal generatorfurther includes: an amplifier configured to receive a second referencesignal and generate the second signal based at least in part on thesecond reference signal.
 43. The system controller of claim 41, furthercomprising: a first switch coupled to the capacitor and configured to beclosed to pass the ramp current in response to the modulation signalbeing at the first logic level; and a second switch coupled to thecapacitor and configured to be closed to pass the second signal inresponse to the modulation signal being at the second logic level. 44.The system controller of claim 32, further comprising: a demagnetizationdetector configured to receive a feedback signal associated with anoutput signal of the power conversion system and generate a triggersignal based at least in part on the feedback signal, the trigger signalindicating an end of a demagnetization process of the power conversionsystem; and a logic controller configured to receive the trigger signaland generate a second signal based at least in part on the triggersignal to affect the drive signal.
 45. The system controller of claim44, further comprising: a current sensing component configured toreceive a current sensing signal associated with the current andgenerate a third signal based at least in part on the current sensingsignal; wherein the demagnetization detector is further configured togenerate a demagnetization signal associated with the demagnetizationprocess based at least in part on the feedback signal; and an erroramplifier configured to: in response to the demagnetization signal beingat a first logic level, receive the third signal and generate a fourthsignal indicating a difference between the third signal and a secondreference signal; and in response to the demagnetization signal being ata second logic level, receive a ground voltage and generate the fourthsignal indicating a difference between the third signal and the groundvoltage.
 46. The system controller of claim 32 wherein the rampingsignal increases in magnitude at a ramping slope during the on-timeperiod.
 47. A method for regulating a power conversion system, themethod comprising: generating a drive signal associated with a switchingperiod including an on-time period and an off-time period; andoutputting the drive signal to a switch to affect a current flowingthrough a primary winding of a power conversion system; wherein theoutputting the drive signal to the switch to affect the currentincludes: outputting the drive signal to close the switch during theon-time period; and outputting the drive signal to open the switchduring the off-time period; wherein a duty cycle is equal to a durationof the on-time period divided by a duration of the switching period;wherein the generating the drive signal associated with the switchingperiod includes keeping a multiplication product of the duty cycle andthe duration of the on-time period approximately constant.
 48. Themethod of claim 47 wherein the keeping the multiplication product of theduty cycle and the duration of the on-time period approximately constantincludes keeping the multiplication product of the duty cycle and theduration of the on-time period constant.
 49. A method for regulating apower conversion system, the method comprising: receiving a modulationsignal; generating a ramp current based at least in part on themodulation signal; receiving the ramp current; generating a rampingsignal based at least in part on the ramp current; receiving the rampingsignal; generating the modulation signal based at least in part on theramping signal; receiving the modulation signal; generating a drivesignal based at least in part on the modulation signal, the drive signalbeing associated with a switching period including an on-time period andan off-time period; and outputting the drive signal to a switch toaffect a current flowing through a primary winding of a power conversionsystem; wherein the outputting the drive signal to the switch to affectthe current includes: outputting the drive signal to close the switchduring the on-time period; and outputting the drive signal to open theswitch during the off-time period; wherein a duty cycle is equal to aduration of the on-time period divided by a duration of the switchingperiod; wherein the generating the ramp current based at least in parton the modulation signal includes generating the ramp currentapproximately proportional to the duty cycle in magnitude.
 50. Themethod of claim 49 wherein the generating the ramp current approximatelyproportional to the duty cycle in magnitude includes generating the rampcurrent proportional to the duty cycle in magnitude.
 51. A method forregulating a power conversion system, the method comprising: providing acompensation signal based on at least information associated with acurrent flowing through a primary winding of a power conversion system;receiving a modulation signal, the compensation signal and a firstreference signal; generating a ramp current based at least in part onthe modulation signal, the compensation signal and the first referencesignal; receiving the ramp current; generating a ramping signal based atleast in part on the ramp current; receiving the ramping signal and thecompensation signal; generating the modulation signal based at least inpart on the ramping signal and the compensation signal; receiving themodulation signal; and outputting a drive signal to a switch to affectthe current, the drive signal being associated with a switching periodincluding an on-time period and an off-time period; wherein theoutputting the drive signal to the switch to affect the currentincludes: outputting the drive signal to close the switch during theon-time period; outputting the drive signal to open the switch duringthe off-time period; and wherein a duty cycle is equal to a duration ofthe on-time period divided by a duration of the switching period;wherein the generating the ramp current based at least in part on themodulation signal, the compensation signal and the first referencesignal includes generating the ramp current approximately proportionalin magnitude to a multiplication product of the duty cycle and adifference, the different representing the first reference signal minusthe compensation signal in magnitude.
 52. The method of claim 51 whereinthe generating the ramp current approximately proportional in magnitudeto the multiplication product of the duty cycle and the differenceincludes generating the ramp current proportional in magnitude to themultiplication product of the duty cycle and the difference.